The present invention relates to video systems, specifically a videophone device which can display video images while the data representing the image displayed is being received from a standard telephone line or from any data channel with slow access time.
Videophones are generally considered complex and expensive systems. A videophone allows the transmission of video images over standard telephone lines. A fundamental problem associated with such transmission is the excessive frequency bandwidth of the video signal, as compared to the bandwidth of the telephone line.
The standard two-wire telephone-set connection, referred to as the public switched telephone network (PSTN), was originally designed to exchange voice between two or more remote users, with a relatively limited bandwidth of approximately 3.0 kHz (300 Hz to 3.3 kHz). This narrow bandwidth allocation was selected to allow more subscribers to simultaneously communicate through the PSTN, with negligible degradation of speech clarity.
Standard NTSC (National Television Systems Conference) video, on the other hand, has a bandwidth of approximately 4 Mhz, which is more than one thousand times the channel bandwidth available on the PSTN.
It is clear that video communication requires a relatively wideband channel, and telephone lines were not designed for this purpose. In general, transmitting an arbitrary signal through the standard telephone line is not possible without xe2x80x9csizingxe2x80x9d the signal within the 300 Hz to 3.3 kHz frequency band. One way to accomplish this is to digitize the arbitrary signal (convert it to a stream of binary bits), and then transmit the digital data that results using a modem (modulator/demodulator). Modems can transmit digital data in the form of analog pulses through the essentially analog, band-limited telephone network. At the transmitter end, digital data (a stream of binary bits, 0""s and 1""s) is modulated into analog tones within the restricted bandwidth of the PSTN. At the receiver end, the analog tones are demodulated, the digital data extracted, and the arbitrary signal reconstructed.
The maximum number of digital bits per second or channel capacity (C) that a modem can transmit is limited by the bandwidth (B) and signal-to-noise ratio (S/N) of the physical channel. Shannon-Hartley theorem defines the relationship between C, B, and S/N as: C=B*log 2(1+S/N) bits/s (bits per second). For the PSTN, the channel capacity is approximately 40 kb/s (kilobits per second), assuming B=3.0 kHz and S/N=40 dB. If the sampling rate of the digitizing circuit is faster than the rate at which the modem can transmit data through the analog channel, the signal cannot be transmitted in real time. This case applies precisely to transmission of video signals through the telephone network. The Nyquist sampling rate required to digitize a video signal of 4 Mhz bandwidth is 8M samples/s, whereas the highest data rate achievable by most currently manufactured modems is 56 kb/s. Assuming that each digital sample contains 8 bits, 64 Mb/s (8Mxc3x978) would be required to transmit the video signal in real time. Even if a sophisticated compression algorithm like JPEG (Joint Photographic Experts Group) were used, which can compress video by a factor of about 20:1, a channel capacity of 3.2 Mb/s would still be required, substantially higher than what standard telephone modems can offer.
To circumvent this difficulty, video signals can be processed before compression and transmission. A standard NTSC video signal consists of 30 video frames per second. If the number of frames per second transmitted is reduced to only 1, for example, the data rate will be reduced by a factor of 30. And if one frame is transmitted every 10 seconds, the data rate required will be reduced by a factor of 300. Generally, the low data rate allowed by the telephone line can be approached by sufficiently decreasing the number of frames per second transmitted. However, decreasing the frame rate degrades the quality of moving video images and precludes the transmission of a full motion picture.
Modem computers process video signals in three dimensions, red, blue, and green, each color component generally requiring digital samples at least six bits wide. Although this method requires 18 bits (rather than eight) of data per pixel (video image dot), the transmission speed of video images can be increased by reducing the number of pixels transmitted, which however results in loss of resolution.
Accordingly, several objects and advantages of the present invention are:
(a) to provide an improved video image display system at very low cost, which can display a video image stored in a digital memory while updating the digital memory with new data received from a telephone line or other data channel with slow access time, without the need for a cache memory;
(b) to provide an efficient method to write or read a digital memory at low speed while data stored in such memory representing a video image is read at high speed to display the image on a monitor, without the need for a cache memory;
(c) to provide a video image display system which can be implemented using standard, off-the-shelf parts, and a simple software code stored in a microcontroller; and
(d) to provide a digital video display method compatible with NTSC, PAL, and SECAM video formats.
Further objects and advantages will be apparent from a consideration of the ensuing description and accompanying drawings.